1. Field of the Invention
The present invention relates to a gallium nitride based compound semiconductor device, and more particularly to a gallium nitride based compound semiconductor light emitting device with a reduced operating current and a reduced device resistance, for emitting a laser beam with a basic transverse mode and a generally circular shape of beam section.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
The requirement for development of a gallium nitride based compound semiconductor laser with a reduced operating current and a reduced device resistance, for emitting a laser beam with a basic transverse mode and a generally circular shape of beam section has been on the increase. The semiconductor laser of this type has a high utilization efficiency of laser beam in applications to laser disks and laser printers. The high utilization efficiency of laser beam may realize a desired high laser power on a recording face and a desired high recording speed. The semiconductor laser of this type does not need an additional optical element for shaping a generally circular beam section. The absence of such additional optical element may realize a scaling down of the device and a reduction of the cost.
A gallium nitride based compound semiconductor laser, which emits a laser beam with a wavelength near 400 nanometers, has a focusing spot area which is smaller by three times than that of a red-color semiconductor laser, which emits a red-color laser beam with a wavelength near 650 nanometers. The smaller focusing spot area is advantageous in applying the laser to high density recording optical disks and laser printers.
A conventional gallium nitride based compound semiconductor laser will be described hereinafter with reference to the drawings. FIG. 1 is a fragmentary cross sectional elevation view of an example of the conventional gallium nitride based compound semiconductor laser. This laser is disclosed by S. Nakamura et al. in Applied Physics Letters, vol. 72, 1998, pp. 2014-2016. The conventional gallium nitride based compound semiconductor laser is formed over a GaN substrate 101 which has a thickness of 80 micrometers. An n-type GaN contact layer 102 with a thickness of 3 micrometers is provided over the GaN substrate 101. An n-type In0.1Ga0.9N layer 103 with a thickness of 0.1 micrometer is provided over the n-type GaN contact layer 102.
An n-type super-lattice cladding layer 104 with a total thickness of 1.2 micrometers is provided over the n-type In0.1Ga0.9N layer 103. The super-lattice comprises 240 periods of alternating laminations of an n-type GaN layer with a thickness of 2.5 nanometers and an n-type Al0.14Ga0.86N layer with a thickness of 2.5 nanometers. An n-type GaN optical guide layer 105 with a thickness of 0.1 micrometer is provided over the n-type super-lattice cladding layer 104. An undoped multiple quantum well active layer 106 is provided over the n-type GaN optical guide layer 105. The multiple quantum well active layer 106 comprises 4 periods of alternating laminations of an undoped In0.15Ga0.85N quantum well layer with a thickness of 2 nanometers and an undoped In0.02Ga0.98N barrier layer with a thickness of 5 nanometers.
A p-type Al0.2Ga0.8N layer 107 with a thickness of 20 nanometers is provided over the undoped multiple quantum well active layer 106. A p-type GaN optical guide layer 108 with a thickness of 0.1 micrometer is provided over the p-type Al0.2Ga0.8N layer 107. A p-type super-lattice cladding layer 109 with a total thickness of 0.6 micrometers is provided over the p-type GaN optical guide layer 108. The super-lattice comprises 120 periods of alternating laminations of an n-type GaN layer with a thickness of 2.5 nanometers and an n-type Al0.14Ga0.86N layer with a thickness of 2.5 nanometers. A p-type GaN contact layer 110 with a thickness of 0.05 micrometers is provided over the p-type super-lattice cladding layer 109.
The p-type super-lattice cladding layer 109 has a ridged structure 113 with a stripe-shape top surface having a width of 3 micrometers. The ridged structure 113 is selectively provided over the p-type GaN optical guide layer 108. An SiO2 insulating layer 114 is provided on sloped side faces of the ridged structure 113 and on the top surface of the p-type GaN optical guide layer 108. The SiO2 insulating layer 114 has an opening over the top of the stripe-shape top surface of the ridged structure of the p-type super-lattice cladding layer 109. The p-type GaN contact layer 110 is provided on the top of the ridged structure of the p-type super-lattice cladding layer 109. The SiO2 insulating layer 114 causes a current confinement into the p-type super-lattice cladding layer 109.
The lamination structure of the n-type In0.1Ga0.9N layer 103, the n-type super-lattice cladding layer 104, the n-type GaN optical guide layer 105, the undoped multiple quantum well active layer 106, the p-type Al0.2Ga0.8N layer 107 and the p-type GaN optical guide layer 108 has a step-like side wall 115 which is formed by a selective etching to the lamination structure and a part of an upper region of the n-type GaN contact layer 102. The n-type GaN contact layer 102 has an etched terrace.
A p-electrode 111 extends over the p-type GaN contact layer 110 and the insulating layer 114. An n-electrode 112 is provided on the etched terrace of the n-type GaN contact layer 102. Since the GaN substrate 101 has a high resistivity, the n-electrode 112 is provided in contact with the n-type GaN contact layer 102.
This laser of FIG. 1 emits a laser beam with a wavelength of 393 nanometers, and has an emission threshold current lower than 110 mA of the conventional one. The GaN crystal substrate is lower in crystal defect or dislocation than a sapphire substrate. The low crystal defect or dislocation of the substrate may realize a longer life-time of the laser. A beam shape in a far field of view has an elliptic shape, wherein the emission laser beam has a horizontal radiation angle of 8 degrees in full width at half maximum and a vertical radiation angle of 31 degrees in full width at half maximum. An aspect ratio is an indication of the elliptic shape. The aspect ratio is given by a ratio of the vertical radiation angle to the horizontal radiation angle. The aspect ratio of this laser is about 3.9. This means that the beam shape is a slender elliptic shape.
Another conventional gallium nitride based compound semiconductor laser will be described hereinafter with reference to the drawings. FIG. 2 is a fragmentary cross sectional elevation view of an example of the other conventional gallium nitride based compound semiconductor laser. This laser is disclosed by S. Nakamura et al. in Applied Physics Letters, vol. 39, 2000, pp. L647-L650. The conventional gallium nitride based compound semiconductor laser is formed over a GaN substrate 201 which has a thickness of 150 micrometers. An n-type Al0.05Ga0.95N contact layer 202 with a thickness of 5 micrometers is provided over the GaN substrate 201. An n-type In0.1Ga0.9N layer 203 with a thickness of 0.1 micrometer is provided over the n-type Al0.05Ga0.95N contact layer 202.
An n-type super-lattice cladding layer 204 with a total thickness of 0.9 micrometers is provided over the n-type Al0.05Ga0.09N contact layer 202. The super-lattice comprises 180 periods of alternating laminations of an n-type GaN layer with a thickness of 2.5 nanometers and an n-type Al0.1Ga0.9N layer with a thickness of 2.5 nanometers. An n-type GaN optical guide layer 205 with a thickness of 0.15 micrometer is provided over the n-type super-lattice cladding layer 204. An undoped multiple quantum well active layer 206 is provided over the n-type GaN optical guide layer 205. The multiple quantum well active layer 206 comprises 3 periods of alternating laminations of an undoped In0.05Ga0.85N quantum well layer with a thickness of 4 nanometers and an undoped In0.02Ga0.98N barrier layer with a thickness of 10 nanometers.
A p-type Al0.35Ga0.65N layer 207 with a thickness of 10 nanometers is provided over the undoped multiple quantum well active layer 206. A p-type GaN optical guide layer 208 with a thickness of 0.15 micrometer is provided over the p-type Al0.35Ga0.65N layer 207. A p-type super-lattice cladding layer 209 with a total thickness of 0.6 micrometers is provided over the p-type GaN optical guide layer 107. The super-lattice comprises 120 periods of alternating laminations of an n-type GaN layer with a thickness of 2.5 nanometers and an n-type Al0.1Ga0.9N layer with a thickness of 2.5 nanometers. A p-type GaN contact layer 210 with a thickness of 15 nanometers is provided over the p-type super-lattice cladding layer 209.
The p-type super-lattice cladding layer 209 has a ridged structure 213 with a stripe-shape top surface having a width of 1.8 micrometers. The ridged structure 213 is selectively provided over the p-type GaN optical guide layer 208. An SiO2 insulating layer 214 is provided on sloped side faces of the ridged structure 213 and on the top surface of the p-type GaN optical guide layer 208. The SiO2 insulating layer 214 has an opening over the top of the stripe-shape top surface of the ridged structure of the p-type super-lattice cladding layer 209. The p-type GaN contact layer 210 is provided on the top of the ridged structure of the p-type super-lattice cladding layer 209. The SiO2 insulating layer 214 causes a current confinement into the p-type super-lattice cladding layer 209.
The lamination structure of the n-type In0.1Ga0.9N layer 203, the n-type super-lattice cladding layer 204, the n-type GaN optical guide layer 205, the undoped multiple quantum well active layer 206, the p-type Al0.2Ga0.8N layer 207 and the p-type GaN optical guide layer 208 has a step-like side wall 215 which is formed by a selective etching to the lamination structure and a part of an upper region of the n-type GaN contact layer 202. The n-type GaN contact layer 202 has an etched terrace.
A p-electrode 211 extends over the p-type GaN contact layer 210 and the insulating layer 214. An n-electrode 212 is provided on the etched terrace of the n-type GaN contact layer 202. Since the GaN substrate 101 has a high resistivity, the n-electrode 212 is provided in contact with the n-type GaN contact layer 202.
This laser of FIG. 2 emits a laser beam with a wavelength of 405 nanometers, and has an emission threshold current lower than 23 mA of the conventional one. The GaN crystal substrate is lower in crystal defect or dislocation than a sapphire substrate. The low crystal defect or dislocation of the substrate may realize a longer life-time of the laser. A beam shape in a far field of view has an elliptic shape, wherein the emission laser beam has a horizontal radiation angle of 11.2 degrees in full width at half maximum and a vertical radiation angle of 29.9 degrees in full width at half maximum. An aspect ratio is an indication of the elliptic shape. The aspect ratio is given by a ratio of the vertical radiation angle to the horizontal radiation angle. The aspect ratio of this laser is about 2.7, even the width of the stripe-shape top surface of the ridge structure is made narrow at 1.8 micrometers for allowing a relatively wide horizontal radiation angle. The aspect ratio of about 2.7 means that the beam shape is a slender elliptic shape.
The GaN substrate is more advantageous than the sapphire substrate in view of the desired increase in the device life-time.
Accordingly, it is an object of the present invention to provide a novel gallium nitride based compound semiconductor laser device free from the above problems.
The present invention provides a light-emitting semiconductor device includes an active layer interposed between first-side and second-side cladding layer, and at least one of first-side and second-side optical guide layers. The following four equations are satisfied:
0.15xe2x89xa6h; 
|xxe2x88x92y|xe2x89xa60.02; 
0.02xe2x89xa6xxe2x89xa60.06; and 
0.34xxe2x88x920.49xe2x89xa6d1+2h, 
where xe2x80x9chxe2x80x9d is a total thickness of the first-side and second-side optical guide layers; xe2x80x9cd1xe2x80x9d is a thickness of the first-side cladding layer; xe2x80x9cxxe2x80x9d is a first Al-index of a first AlGaN bulk crystal which has a first refractive index equal to a first averaged refractive index of the first-side cladding layer; and xe2x80x9cyxe2x80x9d represents a second Al-index of a second AlGaN bulk crystal which has a second refractive index equal to a second averaged refractive index of the second-side cladding layer.
The above, and other objects, features and advantages of the present invention will be apparent from the following descriptions.